Effect of thermal history on the rheological behavior of thermoplastic polyurethanes

The effect of thermal history on the rheological behavior of ester- and eth er-based commercial thermoplastic polyurethanes (TPUs) was investigated. H- 1 and C-13 nuclear magnetic resonance (NMR) spectroscopy indicated that the ester-based TPU consisted of 4,4'-diphenylmethane diisocyanate (MDI) and b utanediol (BDO) as hard segments and poly(tetramethylene adipate) as soft s egments, and the ether-based TPU consisted of MDI-BDO as hard segments and poly(oxytetramethylene)as soft segments. The dynamic storage and loss modul i (G' and G ") of specimens, which had been prepared by injection molding a t different temperatures, were monitored at a fixed angular frequency durin g isothermal annealing. It was found that injection molding temperature emp loyed for specimen preparation had a profound influence on the variations o f G' and G " with time observed during isothermal annealing. Measurements w ere taken of the N-H stretching absorption bands in the Fourier transform i nfrared (FTIR) spectra during isothermal annealing at 170 degrees C for spe cimens prepared by injection molding at different temperatures. The analysi s of FTIR spectra indicated that variations of hydrogen bonding with time d uring isothermal annealing resemble very much variations of G' with time du ring isothermal annealing. Isochronal dynamic temperature sweep experiments indicated that the TPUs exhibited hysteresis effect in the heating and coo ling processes. It is concluded that the microphase separation transition o r order-disorder transition in TPU cannot be determined from the isochronal dynamic temperature sweep experiment. It was observed that plots of log G' versus log G " varied with temperature over the entire range of temperatur es (110-190 degrees C) investigated, suggesting that the morphological stat e of the TPU specimens varied with temperature. Little evidence was found f rom H-1 and C-13 NMR spectroscopy that exchange reactions took place in the TPU specimens during isothermal annealing at elevated temperatures.